What makes an onion work?
Cut it, and it explodes with strong smells, hot flavours, and irritating vapours. But detonate the firepower, and it can be meek and mild - even sweet. The key is to understand an onion's structure.
With this knowledge, any cook can manipulate this versatile bulb to achieve pleasingly specific results.
Onion structure and onion chemistry
Onions come in many different colours and sizes, but all are the same species. The dry bulbs, the vegetable we're exploring here, are 89 percent water and 8 to 9 percent soluble sugars; the rest is minerals, fats, proteins, and sulphur compounds.
The sulphur compounds produce distinctive flavour and an aroma that can be difficult to disguise once you've eaten or touched onions. These oil-soluble compounds easily remain in the oils on your skin. They're also blood-soluble and so can be detected in breath and perspiration.
Some onion compounds do not form until you break cell membranes. As soon as a cell is broken, as by peeling, bruising, or cutting, a definite sequence of reactions follows. A key component is the enzyme alliinase. The degree to which this enzyme is activated affects the intensity of the response.
First, sulphur-containing compounds are produced; these are responsible for irritating vapours, one of which stimulates a tear response, and for a biting astringency. These compounds quickly break down into others, which ultimately yield the onion flavour and aroma. In raw or partially cooked onions, these compounds mask sugars and dominate taste.
How to control onion behaviour
Cutting or breaking the cell membranes develops compounds associated with flavour, aroma, and bitterness. With more disruption, more of these compounds, including lachrymator (tear producer), will be formed. To minimize this effect, peel onions under running water so you rinse away vapours and lachrymator as they are created. Chilling onions before you cut also slows the release of tear producer.
When you chop onions in a food processor, more cells are bruised than when you mince onions by hand--so the flavour is stronger and more bitter.
Cooking time and heat intensity both affect flavour. A short period of high heat brings out strong onion characteristics more quickly. But long cooking over low heat diminishes the strong taste, enhancing the onion's natural sweetness.
Using too high a temperature for too long a time develops bitterness, which is somewhat different from a burnt flavour.
Saute: most flavour (not hot)
Sauteing or stir-frying cut onion over medium-high heat brings out the most flavour. The high temperature volatilizes the first set of compounds, this speeds production of other compounds associated with onion flavour and aroma. Sugars in the onion caramelize, which lends colour and flavour to sauces and stocks.
Slow-cook: sweet, mild, limp
Slow-cook sliced onions in oil over moderate heat until they become very limp and golden. Slicing brings forth flavour components; long gentle cooking dissipates them, unmasking natural sweetness. Some of the soluble onion sugars caramelize during cooking, deepening and enriching flavour.
Whole: mellow and mild
If onion is cooked uncut, hot flavour from its enzyme, alliinase, never develops; you get only mild onion flavour with slight sweetness.
Boil whole: sweet, mild
When small, whole, peeled onions are boiled in water, the onion produces a compound much sweeter than sugar. As with baked whole onions, alliinase's hot flavour doesn't develop.
Deep-fry: crisp, sweet, golden
Cut onion into thin shreds, lightly dust with flour, and fry in a generous amount of oil. The large surface area exposes sulphur compounds. The hot oil drives off many of them and much of the onion's moisture, leaving crisp sweet shreds.
Now you all know about onions!
SnS ;D
